1. Field of the Invention
The present invention relates to a video camera (or a television camera) for converting a picked-up optical image into video (or television) signals.
2. Description of the Prior Art
Conventionally, commercial video cameras have been mostly of so called single plate type in which a color picture image is obtained using a sheet of solid state image pickup device. FIG. 2 shows a basic circuit structure of a single plate camera. An image of an object 1 is focused on an image pickup device 21 through a camera lens 2. The image pickup element 21 is driven by an image pickup device drive part 22 to perform photoelectric conversion of the image into electric charges and output an electric signal after accumulating the charges for a given period of time. This output signal is amplified by a preamplifier 23, and subjected to necessary processing such as gamma correction, separation and synthesis of luminance signal and chrominance signal and addition of synchronizing signal in a process circuit 24 to become a video signal E.sub.VIDEO suitable for a broadcasting system. A control circuit part 25 applies a timing pulse Pt for controlling storage and read-out (output) timings of the image pickup device 21 to the drive part 22, and a synchronizing signal P.sub.SYNC to the process circuit 24. In case of the NTSC system, the vertical synchronizing frequency (field frequency) is 60 Hz, and the horizontal synchronizing frequency is approximately 15.75 KHz.
The picture quality of such commercial video camera as above is not satisfactory. In particular, the SN ratio (signal/noise ratio) of the image pickup device is determined by the structure and the noise characteristic of the image pickup device, and is difficult to the improved sufficiently. Also, the dynamic range of the image pickup device determined by the saturated luminous energy level and the noise level has a great influence upon the latitutde of the video camera. The latitude of a video camera in which a solid state image pickup element is employed is as low as approximately .+-.1 EV (Exposure Value). Hence, for an object having a big contrast difference, the video signal is partly saturated, or black collapsing, lack of chrominance signal and so forth are produced, thereby damaging the picture quality badly.
Such circumstances will be described referring to FIG. 3. An object A of high contrast has a high luminance portion B (the luminance has a gradient in vertical direction) in a horizontal stripe form at the central portion as shown in FIG. 3 (a), and low luminance portions C, D above and below the portion B. It is assumed that only a luminance signal is considered as a video signal for convenience sake, and also that only one field period prescribed in the broadcasting system is considered. In FIG. 3 (b), a profile of the luminance level at the time of vertical scanning of the object A is shown. The axis of abscissa shows the time corresponding to vertical scanning (tv: effective period of vertical scanning) of the video camera, the axis of ordinate shows the luminance amplitude of the object, and Smax, Smin and Sav show the maximum value, the minimum value and the average value of the luminance amplitude, respectively.
It is now considered that the stop value of the camera lens is determined based on an average luminance level (Sav in FIG. 3 (b) in one field. When the contrast of the object A to be picked up is big, and a part of the luminance level of the high luminance portion B exceeds the latitude of the image pickup device of the video camera, the image pickup device outputs a partly saturated luminance signal as shown in FIG. 3 (c) (E: saturation level of the image pickup element).
Further, in case the stop value of the camera lens is determined based on the maximum value (Smax in FIG. 3 (b) of the luminance level in one field, no signal saturation is produced as shown in FIG. 3 (d), but the levels of the low luminance portions C, D that occupy a large part of the picture become very low under the influence of the high luminance portion B and the grading of the low luminance portion is not reproduced (black collapse).
The saturation and black collapse of the video signal described above deteriorate the picture quality extremely, but they happen frequently due to the narrow dynamic range of the image pickup device. This is a big problem in obtaining a high picture quality of the video camera. However, it is very difficult to sufficiently improve the dynamic range of the image pickup device under existing circumstances.
There has been no trial up to now to improve the dynamic range of a video camera from the aspect of drive method and signal processing method for an image pickup device.
In U.S. Pat. No. 4,677,464, a video camera having a construction which is partly related to the present invention is disclosed. In this U.S. Patent, a high speed image pickup system is disclosed, which has made the picture blurring produced in photographing a moving body to be 1/3 of the conventional case by means of a combination of a camera which shoots at a speed 3 times as fast as the existing broadcasting system and a video tape recorder which is constructed so as to record at a speed three times as high as a standard speed. The abovementioned picture blurring is generated because the video signal obtained from the image pickup device corresponds to a photograph taken at a shutter speed corresponding to the vertical scanning period of the broadcasting system in each picture unit (in case of the NTSC system, to a photograph taken with a low speed shutter at 1/60 second).
The basic point of the video camera disclosed in U.S. Pat. No. 4,677,464 is to scan the image pickup device at a speed of integer (M) times of normal scanning speed, thereby to obtain video images M times the existing system, and are composed so that the signals which are output from the image pickup device are processed in process circuits installed in the number M times as many as the number in a normal video camera. Thus, it has no relation whatever with the improvement of picture quality of a video camera (in particular, improvement of the dynamic range) which will be described in detail hereafter in the present invention.